This invention relates to liquid metal ion sources, and, more particularly, to alloys used to evaporate multiple ionic species from such sources.
Liquid metal ion sources provide high current density beams of metallic ions from a source having a small virtual source size. Such high current and small source size are required when the ion beam is to be focused with a high resolution of, for example, less than 1 micrometer spot size and utilized in applications such as fabrication of semiconductor microcircuits by ion implantation. The high current density and small virtual source size are achieved by emitting the ions from a substrate having a sharp point, such as the point of a needle. In one such approach, a needle is covered with a layer of liquid ion source metal, and a cusp in the liquid metal at the point of the needle is created by the application of an electrostatic extraction field. The ions are emitted from this tiny cusp. As the ions are emitted and the amount of liquid alloy decreases, more liquid metal flows from a reservoir down the needle to the cusp to replenish that emitted.
In this type of ion source, a species to be implanted typically resides in a liquid alloy while in the reservoir and on the needle. This alloy must be heated to at least its melting point and remain in the molten state for long periods of time during ion implantation runs. When an alloy is held molten for this long period of time, species which have high vapor pressures can be lost from the alloy in significant amounts, so that the alloy composition changes over time. This change in the composition of the ion source alloy over time can be highly significant and deleterious in the fabrication of semiconductor microcircuits, due to the change in the current density of the ionic species to be implanted in the semiconductor chip. Additionally, the long period of contact between the molten alloy and the emission elements of the liquid metal ion source, including the reservoir and the needle substrate, can cause corrosion and failure of these elements. The lifetime of a liquid metal ion source is often limited by the attack and corrosion of the emission elements by the molten alloy, and such corrosion can undesirably change the emission characteristics of an operating ion source over time.
The most straightforward approach to providing an evaporation source for an ionic species is to provide the species in its elemental, unalloyed form. However, many important dopant and metalloid ions for implantation into active areas of microcircuits, such as arsenic, antimony and phosphorus, have high vapor pressures at their melting points, resulting in atomic evaporation and loss of the element. The melting points are also rather high, so that corrosion of the evaporation elements occurs when the pure liquid metal and the evaporation element are in contact for long periods of time.
An alternative approach is to form an alloy of the desired ion evaporation species with other metal or metalloid constituents chosen so that the melting point of the alloy is lowered below that of the pure species, and further so that the corrosion of the emission elements by the liquid alloy is reduced, as compared with the unalloyed pure evaporation species. Conventionally, the alloy has been chosen to be of eutectic or near-eutectic composition. A eutectic reaction depresses the liquidus temperature of any of the reactants, to an intermediate melting point, which is the composition at which that liquid can exist to the lowest temperature without formation of any solid. The use of a eutectic or near-eutectic composition in a liquid metal ion source allows the source to be operated with the liquid alloy at a minimum temperature, thereby reducing the corrosion rate of the alloy on the evaporation source elements. Both the desired species and the alloying elements are ion evaporated from the source, but the desired species may be selected for implantation using a velocity filter which acts as a mass separator to pass only the selected species.
A further important consideration in the selection of liquid metal ion source alloys is the wetting of the source elements by the alloy. The alloy must wet the evaporation elements sufficiently so that it forms a liquid layer on the evaporation elements, and so that additional metal can flow from the reservoir to the needle tip during continuous evaporation runs. The attainment of sufficiently good wettability and minimization of corrosion are difficult to achieve simultaneously in many instances.
In the fabrication of semiconductor microcircuit devices using ion implantation techniques, it would often be desirable to be able to ion evaporate useful currents of several different ionic species from a single source. Such multiple evaporation capability would allow the sequential or simultaneous implantation of different ionic species into an area of a device, with extreme accuracy and control of lateral and vertical implantation profiles, since neither the target nor the ion source would be changed, replaced or physically adjusted during the process. Self-aligned implantations and laterally profiled dopants could then be routinely fabricated, thus allowing the fabrication of innovative device structures.
For the fabrication of silicon-based microcircuits, it would be particularly useful to have an ion source capable of simultaneously evaporating arsenic ions for n-type shallow, heavy implants, and boron for p-type implants. It would also be desirable to have a source capable of imlanting arsenic and boron ions, and in addition implanting phosphorus ions for n-type deep implants. There have been proposed no ion sources capable of simultaneously ion evaporating arsenic, boron and phosphorus, and previously reported sources evaporating both boron and arsenic had unacceptably low yields of one species and/or short lifetimes. Sources are known to ion evaporate individual ions, such as arsenic, but no sources have been proposed for simultaneously evaporating the combinations of ions indicated above. Experimental and theoretical studies have indicated that simultaneous, continuous evaporation of multiple ionic species of interst would be difficult or impossible, since the ions inherently exhibit different evaporation threshold values. Thus, for example, the evaporation threshold voltage for boron ions is approximately twice that of arsenic ions, and prior work such as disclosed in U.S. Pat. No. 4,367,429 suggests that simutltaneous evaporation of such differing species would not occur. Finally, in an attempt to provide an ion source for simultaneously evaporating multiple species, the usual considerations of wetting and inhibition of corrosion of the emission elements must be satisfied in a more complex alloy.
Consequently, there has been provided no practical ion source for the simultaneous evaporation of multiple ionic species of interest, including arsenic-boron, arsenic-phosphorus, and arsenic-boron-phosphorus. The sources and alloys used therein would desirably allow the operation of the source under stable operating conditions for long periods of time without adverse corrosive effects. The present invention provides such an ion source, and further provides related advantages.